For our “high carbon” steel blades, HOCK TOOLS uses
AISI (American Iron and Steel Institute) O1, a simple high carbon
tool steel with very little added to the iron/steel alloy other
than 1.1% manganese. That pinch of Mn allows the steel to harden
with an oil quench (the “O” in O1 stands for Oil.) Oil
removes heat more slowly than water, reducing the thermal shock
that occurs when orange-hot steel is plunged into water. Reducing
that thermal shock minimizes the risk of cracking or distortion
in the hardened piece. With an alloy as simple as O1, containing
so few alloying elements, the hardened grain structure is as fine
as possible which allows honing to the sharpest possible edge.
AISI A2 differs from O1 with the addition of 5% chromium and 1.1%
molybdenum, allowing it to quench in still air (“A”
for Air.) While “stainless” amounts of chromium (12%
or more) make tool steel “gummy” and hard to sharpen,
the modest amount of chromium in A2 improves its toughness and abrasion
resistance, but imparts only a slight measure of corrosion resistance
(like high carbon steel, it will rust and appropriate preventative
care must be taken.) But there is a trade off. During heat treatment
the chromium addition combines with some of the carbon in the alloy
to form chromium carbides – tough, hard particles dispersed
through the steel. These carbides are the primary contributors to
A2’s celebrated edge retention. However, during heat treatment,
the chromium carbides can grow quite large – large enough
to affect your ability to hone the edge as close to zero-radius
as you may want. And these carbides are held in place with less
strength than the rest of the steel matrix which can allow them
to pop out under the stress of honing or cutting leaving a small
gap in the edge. To strengthen the edge we recommend a larger bevel
angle for A2 than we would use for O1. For a bench plane iron, try
your A2 blade at about 30° or 33°. A chisel or block plane
blade can be even steeper; try 35° or so and see if edge retention
A2 is one of the steels that respond well to cryogenic treatment.
This extreme cold treatment (-320°F) essentially finishes the
original quench, increasing the steel’s toughness without
any decrease in hardness. You get increased wear resistance without
any increase in brittleness so a cryogenically treated blade will
hold its edge longer. You can keep working instead of sharpening.
A2 is a great steel that offers a real improvement in edge retention.
O1, on the other hand, is still preferred by many for its relative
ease of sharpening and its ability to get sharper.